Optical signal quality monitoring devices employing asynchronous sampling have been known as those requiring no clock extraction.
In these optical signal quality monitoring devices employing asynchronous sampling, an evaluation was made for an intensity distribution of the optical signal on the basis of asynchronous eye patterns (refer to Patent Document 1, for example) or evaluation for the preponderant portion of the eye opening by measuring signal eye patterns with asynchronous sampling without using clock axes (refer to Patent Document 2, for example). In these optical signal quality monitoring devices employing asynchronous sampling, it is theoretically possible to provide an optical signal quality monitoring not according to a signal bit rate of the optical signal, because of adoption of asynchronous sampling.
Patent Document 1: Japanese Patent Application Laid-open No. 11-223575 (page 5 through 11, FIGS. 3, 6 and 19)
Patent Document 2: Japanese Patent Application Laid-open No. 2004-48688 (page 8 through 15, FIGS. 4 and 8)